I. Field of the Invention
The present invention relates to semiconductor diode lasers and more specifically to temperature-stabilized diode lasers.
II. Background Art
In order to handle the increasing demands for high data rate communications, fiber-optic databus systems require the ability to multiplex wavelengths. The practical range within which wavelengths can be multiplexed is limited by the luminescence spectrum of semiconductor diode lasers, which has a useful band of only 200-300 .ANG.. It is not practical to fabricate a large number of lasers with different compositions to broaden this range to 2,000 to 10,000 .ANG. so as to allow large numbers of channels without temperature-stabilized wavelengths. Therefore, in order to propagate the greatest number of signals within this narrow spectrum, the wavelengths must be very stable and tightly controlled.
The best wavelength stabilized diode lasers available today are the distributed feedback laser (DFB) and distributed Bragg reflector laser (DBR). In these lasers, rather than using end mirrors, light is reflected from corrugated waveguides (Bragg gratings) back into the laser active area.
Early DFB and DBR lasers used waveguides which were formed from the same material as the active region. Since the material of the active region was chosen to optimize gain properties, not grating properties, some of the grating characteristics were compromised, including temperature stability (due to changes in refractive index with temperature).
The range of operating temperatures over which diode lasers are normally operated is about 20.degree. C. to 70.degree. C. The variation in wavelength is about 5 .ANG./.degree.C. for laser diodes with conventional cavities, and about 1 .ANG./.degree.C. for DFB or DBR lasers. Therefore the wavelength will drift about 50 .ANG. over the required temperature range for even the best stabilized lasers.
Thermally-stabilized DBR lasers were described by S. A. Gurevich, et al., Sov. Tech. Phys. Lett. 11(5), May, 1985 and by Zh.I. Alferov, et al., IEEE Journal of Quantum Electronics, Vol. QE-23, No. 6, June, 1987. The lasers described incorporated a corrugated waveguide consisting of multiple dielectric layers, each layer with approximately equal and opposite changes in refractive index of temperature. The effect intended is that there is no net change in refractive index with temperature due to the offset of one layer by another, thus, no resulting change in wavelength occurs.
The monolithic temperature-stabilized lasers of Gurevich, et al. and Alferov, et al. suffer the disadvantage, however, that the processing required to form the waveguide on a single substrate degrades the reliability of the laser facets. After formation of the active layer, the waveguide is formed by selective chemical etching and "micro-cleaving" of the layers, followed by deposition of the dielectric layers. Exposure of the active area to chemical etching compromises the laser's reliability due to cusping or undercut of the laser active area. It is also probable that the long-term reliability of the facets formed by "micro-cleaving" process is not as good as those formed by conventional cleaving. Further, dielectric layers deposited adjacent to the cleaved facet tend to show "edge effects" in that the edges of the dielectric are thicker, forming an edge bead and interfering with coupling between the laser and the waveguide. These lasers are capable of maintaining a stable wavelength within the 5 .ANG. band, which is still severely limiting for wavelength multiplexing when the useful laser band is only 200-300 .ANG.. In addition, because of different coefficients of expansion for the semiconductor substrate and the deposited dielectric, there is a risk of the dielectric pealing off the substrate. Such problems are particularly likely to occur at the interface between the micro-cleaved laser facet and the dielectric film.
In a co-pending application (Ser. No. 07/458,155), now U.S. Pat. No. 5,043,991 the inventor discloses a method of stabilizing temperature-induced emission wavelength fluctuation in semiconductor diode lasers by assembling the laser together with a temperature-stabilized Bragg waveguide. The purpose of that procedure is to eliminate temperature-induced variations in wavelength, enabling tight control over wavelengths within 1 .ANG., as necessary for advanced optical communications systems.
The procedure of assembling a hybrid temperature-stabilized Bragg reflector waveguide with a diode laser has many advantages over the prior art monolithically-integrated hybrid diode laser with waveguide, namely independent optimization of materials of each of the two components (laser and Bragg reflector) for their specific purpose and elimination of Bragg grating process steps which degrade the laser active area. However, the stability of the external Bragg reflector is affected by mechanical alignment between the laser and Bragg reflector chip. The thickness of the active layer of the laser is typically about 0.1 micron for a conventional diode laser. This means that the waveguide of the separate chip must be aligned to the active layer to tolerances comparable to the active layer thickness. Even small shifts of less than 0.1 micron in the alignment of the waveguide chip with respect to the laser will result in a change in the operating point. For larger offsets, the coupling will be reduced and single-frequency operation may not be achieved or may be intermittent. In addition, in the case of the temperature-stabilized laser, the light launched into the waveguide must also be coupled back into the laser. This forces a waveguide to be approximately 0.1 micron thick, as well, to maintain efficient coupling in both directions.
An experimental solution to the alignment difficulty of external waveguides is to place the laser and waveguide on separate precision mechanical stages. This, however, is impractical for commercial application where trial-and-error adjustment is contrary to the concept of the off-the-shelf flexibility. See, e.g., J. M. Hammer, et al., Single-Wavelength Operation of the Hybrid-External Bragg-Reflector-Waveguide Laser Under Dynamic Conditions, Appl. Phys. Lett. 47(3) Aug. 1, 1985.
It would be desirable to establish a technique for monolithically providing a temperature-stabilized waveguide which provides the same effect as an external Bragg grating without the alignment difficulties involved with coupling an external waveguide to the laser which is to be stabilized and which does not detract from the lifetime of the laser. It is toward this object that the present invention is directed.